1. Field of the Invention
The present invention relates to a method of manufacturing a substrate having a transparent conductive film, a substrate having a transparent conductive film manufactured using the method, and a touch panel using the substrate.
2. Prior Art
Conventionally, glass substrates having a tin-oxide-containing indium oxide (hereinafter referred to as xe2x80x98ITOxe2x80x99) film are widely used as the substrates having a transparent conductive film of liquid crystal display panels (hereinafter referred to as xe2x80x98LCDsxe2x80x99) and the like. In recent years, such glass substrates having an ITO film have also come to be used in resistive film type touch panels.
As shown in FIG. 3, a resistive film type touch panel is composed of a transparent upper electrode 1 and a lower electrode 3, with the lower electrode 3 and the upper electrode 1 being arranged in facing relation to one another with spacers 2 therebetween; when the upper electrode 1 is pushed with a pen 4, a finger or the like, the upper electrode 1 comes into contact with the lower electrode 3.
The method of detection of the input position in such a resistive film type touch panel may be analog or digital, with the method selectively used depending on the application. The analog detection method, which gives a higher detection resolution, is used in a wider range of applications. In the case of an analog detection type touch panel, a voltage is applied between two parallel electrodes, one at each end in a prescribed direction of the touch panel, as shown in FIG. 4A, thus forming a linear potential gradient as shown in FIG. 4B, and by measuring the voltage Ec corresponding to an input contact point, the input position c is detected.
It is required that such a resistive film type touch panel transmits light well, and so the ITO film formed on the substrate is extremely thin, usually not more than 30 nm (see for example Japanese Laid-open Patent Publication (Kokai) No. 11-167827). Moreover, in the case of an analog detection type touch panel, the ITO film on the glass substrate usually has a surface resistance of 300 to 2000xcexa9, and moreover since this ITO film is used as the electrode for determining the input position, there is a strict requirement for the surface resistance to be uniform.
The steps in manufacturing a resistive film type touch panel are to subject a lower electrode 3 comprised of a glass substrate having an ITO film to resist printing, etching, silver ink printing, insulating ink printing, micro dot spacer formation, scribing and so on, next bond the lower electrode 3 to an upper electrode 1 that has been prepared in a separate manufacturing step, and then seal around the upper electrode 1 and the lower electrode 3 by heat sealing. Of these steps, in a printing step such as silver ink printing or insulating ink printing and in the heat sealing step, heat treatment at a maximum temperature of 160xc2x0 C. is carried out, and during this heat treatment the surface resistance of the ITO film changes; as a result, in some cases a touch panel manufactured using such a substrate will not function properly.
Specifically, when the surface resistance of the ITO film has dropped due to heat treatment, the detected voltage drops and thus a discrepancy arises between the input position and the detected position; when the surface resistance has risen, the detected voltage rises and thus the same kind of discrepancy again becomes prone to occur. Moreover, if the surface resistance distribution is not uniform, then the potential gradient loses its linearity as shown in FIG. 4B, that is, there is no longer a linear relationship between distance and potential, and hence it becomes impossible to detect the input position accurately.
A description will now be given, with reference to FIG. 5, of the reason why heat treatment causes the surface resistance of an ITO film to change.
The ITO film is polycrystalline, and the more film deposition is carried out at a high temperature above about 160xc2x0 C., which is the crystallization temperature of ITO, the more crystallization proceeds and the more stable the crystal structure becomes. However, impurity-containing ITO and non-crystalline ITO, for which the crystal structure changes easily during subsequent heat treatment, are inevitably formed near the substrate surface during the initial stage of the film deposition (FIG. 5A), and in particular since the lattice constant of ITO is high, i.e. about 1 nm, at a film thickness of less than 30 nm the proportion of crystallized ITO is low and the proportion of impurity-containing ITO and non-crystalline ITO high (FIG. 5B). When such an ITO film containing a lot of impurity-containing ITO and non-crystalline ITO is subsequently subjected to heat treatment, the crystal structure changes due to the progression of crystallization of the film and the occurrence of oxidation, and hence the electrical properties, and thus the surface properties, of the ITO film change. This change in the surface properties becomes greater as the ITO film is made thinner and hence the proportion of the crystal structure that is unstable increases (FIG. 5A).
Considering the effects of the heat treatment that must inevitably be carried out during the manufacturing of a touch panel, it is desirable for the percentage change between the surface resistance of the ITO film before heat treatment and the surface resistance of the ITO film after heat treatment to be not more than xc2x110%.
The ITO film is formed on the surface of the glass substrate using an in-line type or batch type sputtering device that uses magnetron sputtering (hereinafter referred to merely as xe2x80x98sputteringxe2x80x99).
With such an existing sputtering device, it is difficult to keep the above-mentioned percentage change in the surface resistance of the ITO film down to no more than xc2x110% under typical conditions used conventionally when manufacturing a transparent conductive film for an LCD (for example, substrate temperature: 200 to 400xc2x0 C.; sputtering pressure: 0.27 to 1.33 Pa; ratio of oxygen to argon in the mixed gas: 0 to 0.04), particularly when the thickness of the ITO film is less than 30 nm.
Methods proposed in the past for reducing the percentage change in the surface resistance have been, in the case of sputtering, to make the nitrogen doping amount in the ITO film 0.01 to 0.6 mass % and the film thickness 5 to 25 nm or to use no nitrogen doping in the ITO film and make the tin doping amount 4.2 to 8.3 at % (see Japanese Laid-open Patent Publication (Kokai) No. 2000-113732), and, in the case of applying a coating solution that forms an ITO film upon pyrolysis, to apply two such coating solutions having different pyrolysis temperatures onto the glass substrate, and then, to improve the stability of the resistance to heat treatment, carry out baking first under an oxidizing atmosphere and then under a non-oxidizing atmosphere (see Japanese Laid-open Patent Publication (Kokai) No. 2000-100263).
However, in the case of the former of the above methods, it was found in 200xc2x0 C. 60-minute heat resistance tests that the percentage change in the surface resistance was kept down to within xe2x88x9210% to +19%, but there is a problem that this is still insufficient. Moreover, in the case of the latter of the above methods, there is a problem in that during the application of the coating solutions it is impossible to avoid foreign particles getting into the coating solutions and then during the baking there is a high probability of these foreign particles sticking, and a problem in that the percentage change in the surface resistance cannot be kept down to within xc2x120%, which is insufficient.
It is an object of the present invention to provide a method of manufacturing a substrate having a transparent conductive film, which is capable of suppressing change in the surface resistance of the film upon the substrate being subjected to prescribed heat treatment, a substrate having a transparent conductive film manufactured using the method, and a touch panel using the substrate.
The present inventors conducted assiduous studies to attain the above object, and as a result discovered that, in the case of a method of manufacturing a substrate having a transparent conductive film in which sputtering is carried out on a transparent insulating substrate using an indium oxide/tin oxide target under an atmosphere of a mixed gas containing argon and oxygen, if the ratio of oxygen to argon in the mixed gas is set such that the carrier density of the film becomes a maximum, then crystallization of the film is promoted until a stable state as a conductive film is reached and change in the surface resistance of the film upon the substrate being subjected to prescribed heat treatment is suppressed, and in particular that if the ratio of oxygen to argon in the mixed gas is in a range of 0.016 to 0.018, then the percentage change between the surface resistance of the film before the prescribed heat treatment and the surface resistance of the film after the prescribed heat treatment can be kept down to within xc2x110%.
To attain the above object, the present invention provides a method of manufacturing a substrate having a transparent conductive film, by carrying out sputtering on a transparent insulating substrate using an indium oxide/tin oxide target under an atmosphere comprising a mixed gas of argon and oxygen, wherein a ratio of oxygen to argon in the mixed gas is set such that the film has a maximum carrier density.
According to this method of manufacturing a substrate having a transparent conductive film, the ratio of oxygen to argon in the mixed gas is set such that the carrier density of the film becomes a maximum, and hence crystallization of the film is promoted until a stable state as a conductive film is reached and change in the surface resistance of the film upon the substrate being subjected to prescribed heat treatment is suppressed.
Preferably, the ratio of oxygen to argon in the mixed gas is in a range of 0.016 to 0.018.
If the ratio of oxygen to argon in the mixed gas is controlled to the range of 0.016 to 0.08, crystallization of the film is promoted until a stable state as a conductive film is reached and change in the surface resistance of the film upon the substrate being subjected to prescribed heat treatment is suppressed.
The method according to the present invention includes a step of carrying out heat treatment on the transparent insulating substrate after the sputtering, and wherein a percentage change between a surface resistance of the film before the heat treatment and a surface resistance of the film after the heat treatment is within a range of xc2x110%.
According to the method of the present invention, heat treatment is carried out after the sputtering, and the percentage change between the surface resistance of the film before the heat treatment and the surface resistance of the film after the heat treatment is not more than xc2x110%, and hence an appropriate potential gradient with respect to position can be obtained and a substrate suitable for a touch panel can be provided.
According to the method of the present invention, even if heat treatment is carried out under ordinary conditions, that is, for at least 30 minutes and at at least 160xc2x0 C., a substrate suitable for a touch panel can be reliably provided.
Preferably, the method according to the present invention includes a step of removing impurities from a surface of the transparent insulating substrate before the sputtering is carried out.
Since impurities are removed from the surface of the transparent insulating substrate before sputtering, the formation of an impurity-containing ITO film on the surface of the substrate can be suppressed, and an ITO film having improved crystallinity can be obtained.
Also preferably, the film has a thickness of not more than 30 nm. As a result, the optical transparency of the substrate can be improved.
Preferably, the surface resistance of the film is in a range of 300 to 1000xcexa9. As a result, an appropriate potential gradient with respect to position can be obtained and a substrate suitable for a touch panel can be provided.
Preferably, the variation in the surface resistance distribution of the film is not more than xc2x110%. As a result, an appropriate potential gradient with respect to position can be reliably obtained and a substrate suitable for a touch panel can be provided.
Also preferably, the transparent insulating substrate has at least 90% of transmittance of light of wavelength 550 nm after the sputtering. As a result, a substrate suitable for a touch panel that must have high optical transparency can be reliably obtained.
If a glass substrate is used for the transparent insulating substrate, the above-mentioned effects can be realized reliably.
Further, if an SiO2 film is formed on the glass substrate in advance, the above-mentioned effects can be realized more reliably.
To attain the above object, the present invention further provides a substrate having a transparent conductive film manufactured using a method wherein sputtering is carried out on a transparent insulating substrate using an indium oxide/tin oxide target under an atmosphere comprising a mixed gas of argon and oxygen, and wherein a ratio of oxygen to argon in the mixed gas is set such that the film has a maximum carrier density.
According to this substrate having a transparent conductive film, a substrate suitable for a touch panel can be obtained.
To attain the above object, the present invention also provides a touch panel using the above substrate having a transparent conductive film.
According to this touch panel, input positions can be detected accurately.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.